Whenever this subject appears on pprune, as it regularly does, sooner or later someone wearily points out that body fat results from what you stuff down your throat. THERE IS NO OTHER SOURCE.
Ipso facto, it's a lifestyle choice.
And that means that if you're too fat to fit in the seat you paid for, you have to buy another one. You have a range of options
* Don't fly
* Lose weight and save money
* Pay up and stop moaning
Ans spare us all the crap excuses like metabolic rates etc etc. If you eat less you get thinner, if you are an adult. The reference above to concentration camps may have been tasteless, but it proves the point.
If you don't want to be humiliated, don't seek to impose your mass of greasy, smelly body fat on passengers on each side of you. If you do it to me, I'll ask to be moved, or for you to be moved. You have no rights to make my journey hell.